The present invention relates to methods of manufacturing bulk metallic glass compositions, and more particularly to such methods that involve microalloying with impurity-mitigating dopants.
Bulk metallic glasses (BMGs) constitute a new class of metallic materials with attractive properties, for example, extremely high specific strength and unique deformation behavior. BMGs are suitable for many structural and functional applications, including: submarine, ship, aeronautical and aerospace materials, especially for defense industries; die and mold materials for manufacturing industries; recreation materials such as golf club heads, fishing rods, bicycles, etc.; soft magnetic materials for engineering control systems; and, especially, medical instruments. See U.S. patent application Ser. No. 09/799,445 filed on Mar. 5, 2001 by Joseph A Horton Jr. and Douglas E. Parsell entitled xe2x80x9cBulk Metallic Glass Medical Instruments, Implants and Methods of Using Samexe2x80x9d, the entire disclosure of which is incorporated herein by reference.
It is well established that interstitial impurities, such as oxygen and nitrogen, which are generally present in charge materials, have an adverse effect on the critical cooling rate necessary for the formation of glass states in Zr-base BAM systems. In general, oxygen concentrations of about one thousand pairs per million in weight (wppm) are known to reduce the glass forming ability and increase the critical cooling rate of these BAMs by several orders of magnitude. Because of the harmful effect of oxygen, high-purity Zr metal has been required for manufacturing BAM parts with large cross sections. The disadvantage of this approach is that high-purity charge materials are very expensive and substantially increase the material and processing, costs. For instance, the price of commercially pure Zr metal may be in the order of $50 per lb, and greater than $500 per lb for high-purity Zr necessary for producing glass states. Moreover, such an approach requires processing in ultra-clean systems in order to avoid oxygen contamination of BAMs, resulting in the further increase of production cost.
In order to demonstrate the harmful effect of oxygen impurity on the glass forming ability of BMGs, a well known Zr-base BMG alloy, BAM-11, with the composition of 10 at. % Al, 5 at. % Ti, 17.9 at. % Cu, 14.6 at. % Ni, balance Zr, was selected as a model material for study. Two Zr metal sources were chosen for alloy preparation: one was a high-purity (HP) metal containing 560 wppm oxygen and the other was a commercial-pure (CP) metal containing 4460 wppm oxygen. The purchase prices per pound for Zr metal were $54 for Zr (CP) and $546 for Zr (HP). Alloy ingots were prepared by arc melting and drop casting into a copper mold of xc2xcxe2x80x3 diameter.
FIGS. 1a and 1b show back-scattered electron micrographs of these two alloy ingots, respectively: BAM-11 (HP) and BAM-11 (CP). Comparison thereof indicated that the glass phase was formed in BAM-11 (HP) and crystalline phase was formed in BAM-11 (CP) in the central region of the alloy ingots. Thus, the oxygen impurity in CP Zr dramatically and deleteriously reduced the glass forming(g ability of the BMG alloy. Tensile specimens were prepared from these two ingots and tested at room temperature in air. As indicated in Table 1, the oxygen impurity, which suppressed the glass state in the CP material, also reduced the tensile fracture strength of BAM-11 from 1730 MPa for the HP material down to essentially zero for the CP material at room temperature.
The impurity problem must be solved satisfactorily in order to achieve feasibility of BMGs for General engineering use and commercial products at reasonable cost. It is thus vital to develop a new and improved method to manufacture BMGs for commercialization.
Accordingly, objects of the present invention include: neutralization of the harmful effect of interstitial impurities in charge materials used for BMG production so that relatively impure materials can be used to manufacture BMGs economically. Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method of making a bulk metallic glass composition including, the steps of
a. providing a starting material suitable for making a bulk metallic lass composition;
b. adding at least one impurity-mitigating dopant to the starting material to form a doped starting material; and
c. converting the doped starting material to a bulk metallic glass composition so that the impurity-mitigating dopant reacts with impurities in the starling material to neutralize deleterious effects of the impurities on the formation of the bulk metallic glass composition.
In accordance with another aspect of the present invention, a bulk metallic glass composition includes a bulk metallic glass which comprises at least one impurity-mitigating dopant.